Data unit format for multi-user data in long-range wireless local area networks (WLANS)

ABSTRACT

A method includes receiving a data unit that includes a signal (SIG) field and a data field. The SIG field provides information for interpreting the data field. The method also includes detecting a first symbol constellation rotation of at least a first orthogonal frequency division multiplexing (OFDM) symbol in the SIG field of the data unit, determining, based at least in part on the detected first symbol constellation rotation, a number of information bits per OFDM symbol in the SIG field of the data unit, processing the SIG field of the data unit according to the determined number of information bits per OFDM symbol in the SIG field, and processing the data field of the data unit according to the information for interpreting the data field as provided in the SIG field of the data unit.

CROSS-REFERENCES TO RELATED APPLICATIONS

This disclosure is a continuation of U.S. Ser. No. 13/663,106, entitled“Data Unit Format for Multi-User Data in Long-Range Wireless Local AreaNetworks (WLANS),” filed on Oct. 29, 2012, now U.S. Pat. No. 8,942,320,which claims the benefit of U.S. Provisional Patent Application No.61/552,420, entitled “11ah 1 MHz MU Format,” filed on Oct. 27, 2011. Thedisclosures of the applications identified above are incorporated byreference herein in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to communication networks and,more particularly, to long-range wireless local area networks.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

When operating in an infrastructure mode, wireless local area networks(WLANs) typically include an access point (AP) and one or more clientstations. WLANs have evolved rapidly over the past decade. Developmentof WLAN standards such as the Institute for Electrical and ElectronicsEngineers (IEEE) 802.11a, 802.11b, 802.11g, and 802.11n Standards hasimproved single-user peak data throughput. For example, the IEEE 802.11bStandard specifies a single-user peak throughput of 11 megabits persecond (Mbps), the IEEE 802.11a and 802.11g Standards specify asingle-user peak throughput of 54 Mbps, the IEEE 802.11n Standardspecifies a single-user peak throughput of 600 Mbps, and the IEEE802.11ac Standard specifies a single-user peak throughput in thegigabits per second (Gbps) range.

Work has begun on two new standards, IEEE 802.11 ah and IEEE 802.11af,each of which will specify wireless network operation in sub-1 GHzfrequencies. Low frequency communication channels are generallycharacterized by better propagation qualities and extended propagationranges compared to transmission at higher frequencies. In the past,sub-1 GHz ranges have not been utilized for wireless communicationnetworks because such frequencies were reserved for other applications(e.g., licensed TV frequency bands, radio frequency band, etc.). Thereare few frequency bands in the sub-1 GHz range that remain unlicensed,with different specific unlicensed frequencies in different geographicalregions. The IEEE 802.11ah Standard will specify wireless operation inavailable unlicensed sub-1 GHz frequency bands. The IEEE 802.11afStandard will specify wireless operation in TV White Space (TVWS), i.e.,unused TV channels in sub-1 GHz frequency bands.

SUMMARY

In one embodiment, a method includes receiving a data unit that includesa signal (SIG) field and a data field. The SIG field providesinformation for interpreting the data field. The method also includesdetecting a first symbol constellation rotation of at least a firstorthogonal frequency division multiplexing (OFDM) symbol in the SIGfield of the data unit, determining, based at least in part on thedetected first symbol constellation rotation, a number of informationbits per OFDM symbol in the SIG field of the data unit, processing theSIG field of the data unit according to the determined number ofinformation bits per OFDM symbol in the SIG field, and processing thedata field of the data unit according to the information forinterpreting the data field as provided in the SIG field of the dataunit.

In other embodiments, the method may comprise one or more (or none) ofthe following elements. Determining a number of information bits perOFDM symbol in the SIG field may include determining one or more of (i)a modulation type of the SIG field, (ii) a coding rate of the SIG field,and (iii) a number of bit repetitions of the SIG field, and processingthe SIG field of the data unit according to the determined number ofinformation bits per OFDM symbol in the SIG field may include processingthe SIG field of the data unit according to the determined one or moreof (i) the modulation type, (ii) the coding rate, and (iii) the numberof bit repetitions. Detecting a first symbol constellation rotation mayinclude detecting whether at least the first OFDM symbol in the SIGfield of the data unit is binary phase shift key (BPSK) modulated orquaternary BPSK (QBPSK) modulated. Determining one or more of (i) amodulation type of the SIG field, (ii) a coding rate of the SIG field,and (iii) a number of bit repetitions of the SIG field may includedetermining whether the SIG field includes a first number of bitrepetitions or a second number of bit repetitions, and processing theSIG field of the data unit according to the determined number ofinformation bits per OFDM symbol in the SIG field may include decodingthe SIG field according to the determined number of bit repetitions. Themethod may further comprise determining, based at least in part on (i)the detected first symbol constellation rotation or (ii) a second symbolconstellation rotation of at least a second OFDM symbol within the SIGfield of the data unit, whether at least a portion of a preamble of thedata unit is arranged according to a shorter format or a longer format.Determining whether at least a portion of a preamble of the data unit isarranged according to a shorter format or a longer format may includedetermining whether a portion of the preamble after the SIG field isarranged according to a single-user format or a multi-user format.

In another embodiment, an apparatus includes a network interfaceconfigured to receive a data unit that includes a signal (SIG) field anda data field. The SIG field provides information for interpreting thedata field. The network interface is also configured to detect a firstsymbol constellation rotation of at least a first orthogonal frequencydivision multiplexing (OFDM) symbol in the SIG field of the data unit,determine, based on the detected first symbol constellation rotation, anumber of information bits per OFDM symbol in the SIG field of the dataunit, process the SIG field of the data unit according to the determinednumber of information bits per OFDM symbol in the SIG field, and processthe data field of the data unit according to the information forinterpreting the data field as provided in the SIG field of the dataunit.

In other embodiments, the apparatus may comprise one or more (or none)of the following elements. The network interface may be configured todetermine the number of information bits per OFDM symbol in the SIGfield at least in part by determining one or more of (i) a modulationtype of the SIG field, (ii) a coding rate of the SIG field, and (iii) anumber of bit repetitions of the SIG field, and process the SIG field ofthe data unit according to the determined number of information bits perOFDM symbol in the SIG field at least in part by processing the SIGfield of the data unit according to the determined one or more of (i)the modulation type, (ii) the coding rate, and (iii) the number of bitrepetitions. The network interface may be configured to detect the firstsymbol constellation rotation at least in part by detecting whether atleast the first OFDM symbol in the SIG field of the data unit is binaryphase shift key (BPSK) modulated or quaternary BPSK (QBPSK) modulated.The network interface may be configured to determine one or more of (i)the modulation type of the SIG field, (ii) the coding rate of the SIGfield, and (iii) the number of bit repetitions of the SIG field at leastin part by determining whether the SIG field includes a first number ofbit repetitions or a second number of bit repetitions, and process theSIG field of the data unit according to the determined number ofinformation bits per OFDM symbol in the SIG field at least in part bydecoding the SIG field according to the determined number of bitrepetitions. The network interface may be further configured todetermine, based at least in part on (i) the detected first symbolconstellation rotation or (ii) a second symbol constellation rotation ofat least a second OFDM symbol within the SIG field of the data unit,whether at least a portion of a preamble of the data unit is arrangedaccording to a shorter format or a longer format. The network interfacemay be configured to determine whether at least the portion of thepreamble of the data unit is arranged according to the shorter format orthe longer format at least in part by determining whether a portion ofthe preamble after the SIG field is arranged according to a single-userformat or a multi-user format.

In another embodiment, a method includes determining a first number ofinformation bits per orthogonal frequency division multiplexing (OFDM)symbol to be used in generating a data field of a data unit,determining, based on the first number of information bits per OFDMsymbol, a second number of information bits per OFDM symbol to be usedin generating a signal (SIG) field of the data unit, determining, basedon the second number of information bits per OFDM symbol, a first symbolconstellation rotation, and generating the SIG field of the data unitaccording to the second number of information bits per symbol. The SIGfield provides a receiver with information for interpreting the datafield. The method also includes generating the SIG field of the dataunit includes generating at least a first OFDM symbol according to thefirst symbol constellation rotation, and generating the data field ofthe data unit according to the first number of information bits per OFDMsymbol.

In other embodiments, the method may comprise one or more (or none) ofthe following elements. Determining a first number of information bitsper OFDM symbol to be used in generating the data field may includedetermining the first number of information bits per OFDM symbol basedon channel state information. Determining a second number of informationbits per OFDM symbol to be used in generating the SIG field may includesetting the second number of information bits per OFDM symbol equal tothe first number of information bits per OFDM symbol when the firstnumber of information bits per OFDM symbol is a minimum number ofinformation bits per OFDM symbol. Determining a first number ofinformation bits per OFDM symbol to be used in generating the data fieldmay include determining one or more of (i) a modulation type to be usedin generating the data field, (ii) a coding rate to be used ingenerating the data field, and (iii) a number of bit repetitions to beused in generating the data field, and determining a second number ofinformation bits per OFDM symbol to be used in generating the SIG fieldmay include determining one or more of (i) a modulation type to be usedin generating the SIG field, (ii) a coding rate to be used in generatingthe SIG field, and (iii) a number of bit repetitions to be used ingenerating the SIG field. Determining a first symbol constellationrotation based on the second number of information bits per OFDM symbolmay include selecting binary phase shift key (BPSK) modulation orquaternary BPSK (QBPSK) modulation for the first OFDM symbol of the SIGfield based on the determined one or more of (i) the modulation type tobe used in generating the SIG field, (ii) the coding rate to be used ingenerating the SIG field, and (iii) the number of bit repetitions to beused in generating the SIG field. The method may further comprisedetermining a second symbol constellation rotation based on whether thedata unit is a single-user data unit or a multi-user data unit, andgenerating the SIG field of the data unit may further include generatingat least a second OFDM symbol according to the second symbolconstellation rotation.

In another embodiment, an apparatus includes a network interfaceconfigured to determine a first number of information bits per OFDMsymbol to be used in generating a data field of a data unit, determine,based on the first number of information bits per OFDM symbol, a secondnumber of information bits per OFDM symbol to be used in generating asignal (SIG) field of the data unit, determine, based on the secondnumber of information bits per OFDM symbol, a first symbol constellationrotation, and generate the SIG field of the data unit according to thesecond number of information bits per OFDM symbol. The SIG fieldprovides a receiver with information for interpreting the data field.The network interface is also configured to generate the SIG field atleast in part by generating at least a first OFDM symbol according tothe first symbol constellation rotation, and generate the data field ofthe data unit according to the first number of information bits per OFDMsymbol.

In other embodiments, the apparatus may comprise one or more (or none)of the following elements. The network interface may be configured todetermine a first number of information bits per OFDM symbol to be usedin generating the data field at least in part by determining the firstnumber of information bits per OFDM symbol based on channel stateinformation. The network interface may be configured to determine afirst number of information bits per OFDM symbol to be used ingenerating the data field at least in part by determining one or more of(i) a modulation type to be used in generating the data field, (ii) acoding rate to be used in generating the data field, and (iii) a numberof bit repetitions to be used in generating the data field, anddetermine a second number of information bits per OFDM symbol to be usedin generating the SIG field at least in part by determining one or moreof (i) a modulation type to be used in generating the SIG field, (ii) acoding rate to be used in generating the SIG field, and (iii) a numberof bit repetitions to be used in generating the SIG field. The networkinterface may be configured to determine a first symbol constellationrotation based on the second number of information bits per OFDM symbolat least in part by selecting binary phase shift key (BPSK) modulationor quaternary BPSK (QBPSK) modulation for the first OFDM symbol of theSIG field based on the determined one or more of (i) the modulation typeto be used in generating the SIG field, (ii) the coding rate to be usedin generating the SIG field, and (iii) the number of bit repetitions tobe used in generating the SIG field. The network interface may beconfigured to determine a second symbol constellation rotation based onwhether the data unit is a single-user data unit or a multi-user dataunit, and generate the SIG field of the data unit in part by generatingat least a second OFDM symbol according to the second symbolconstellation rotation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example wireless local area network(WLAN), according to an embodiment.

FIG. 2 is a diagram of example normal mode and low bandwidth mode dataunits, according to an embodiment.

FIG. 3 is a diagram of example single-user and multi-user formats of anormal mode data unit, according to an embodiment.

FIG. 4 is a diagram illustrating example modulation techniques used tomodulate symbols within signal (SIG) fields of a data unit preamble,according to an embodiment.

FIG. 5 is a diagram of example single-user and multi-user formats of alow bandwidth mode data unit, according to an embodiment.

FIG. 6A is a diagram of example SIG fields of low bandwidth mode dataunits, corresponding to single-user and multi-user formats, according toan embodiment.

FIG. 6B is a diagram of example SIG fields of low bandwidth mode dataunits, corresponding to single-user and multi-user formats, according toan alternative embodiment.

FIG. 7A is a diagram of example SIG fields of low bandwidth mode dataunits, corresponding to first and second single-user formats and amulti-user format, according to an embodiment.

FIG. 7B is a diagram of example SIG fields of low bandwidth mode dataunits, corresponding to first and second single-user formats and amulti-user format, according to an alternative embodiment.

FIG. 8 is a flow diagram of an example method for generating a dataunit, according to an embodiment.

FIG. 9 is a flow diagram of an example method for receiving andprocessing a data unit, according to an embodiment.

DETAILED DESCRIPTION

In embodiments described below, a wireless network device such as anaccess point (AP) of a wireless local area network (WLAN) transmits datastreams to one or more client stations. The AP is configured to operatewith client stations according to at least a first communicationprotocol. In an embodiment, the first communication protocol definesoperation in a sub-1 GHz frequency range, and is typically used forapplications requiring long range wireless communication with relativelylow data rates. The first communication protocol (e.g., IEEE 802.11af orIEEE 802.11ah) is referred to herein as a “long range” communicationprotocol. In some embodiments, the AP is also configured to communicatewith client stations according to one or more other communicationprotocols which define operation in generally higher frequency rangesand are typically used for closer-range communications with higher datarates. The higher frequency communication protocols (e.g., IEEE 802.11a,IEEE 802.11n, and/or IEEE 802.11 ac) are collectively referred to hereinas “short range” communication protocols. In some embodiments, physicallayer (PHY) data units conforming to the long range communicationprotocol (“long range data units”) are the same as or similar to dataunits conforming to a short range communication protocol (“short rangedata units”), but are generated using a lower clock rate. To this end,in an embodiment, the AP operates at a clock rate suitable for shortrange operation, and down-clocking is used to generate a clock to beused for the sub-1 GHz operation. As a result, in this embodiment, along range data unit maintains the physical layer format of a shortrange data unit, but is transmitted over a longer period of time.Example formats of long range data units, according to variousembodiments, are described in U.S. patent application Ser. No.13/359,336, “Physical Layer Frame Format For Long Range WLAN,” now U.S.Pat. No. 8,867,653, the disclosure of which is hereby incorporated byreference herein in its entirety.

In addition to this “normal mode” specified by the long rangecommunication protocol, in some embodiments, the long rangecommunication protocol also specifies a “low rate mode” with a data ratecompared to the lowest data rate specified for the normal mode. In someof these embodiments, low rate mode data units are “low bandwidth mode”data units transmitted over a bandwidth less than the lowest channelbandwidth for normal mode data units. For example, low bandwidth modedata units are generated using a 32-point inverse discrete Fouriertransform (IDFT) for transmission over a 1 MHz bandwidth, while normalmode data units are generated using the same clock rate, but a 64-pointor larger IDFT for transmission over a 2 MHz or greater bandwidth isused, in an embodiment. A lower data rate allows the low rate mode tofurther extend communication range, and generally improves receiversensitivity (or sensitivity gain). In various embodiments, for example,the low rate mode is used as a control mode (e.g., for signal beacon orassociation procedures, transmit beamforming training operations, etc.),or as an extension of the normal mode for extended range. Exampleformats of low rate mode data units (including low bandwidth mode dataunits), and the generation of such data units, according to variousembodiments, are described in U.S. patent application Ser. No.13/366,064, “Control Mode PHY for WLAN,” now U.S. Publication No.2012-0201316, and U.S. patent application Ser. No. 13/494,505, “LowBandwidth PHY for WLAN,” now U.S. Pat. No. 8,826,106, the disclosures ofwhich are hereby incorporated by reference herein in their entireties.

In some embodiments, normal mode data units and low rate mode data unitsinclude one or more short training fields (STFs) for packet detectionand automatic gain control, one or more long training fields (LTFs) forchannel estimation, and one or more signal (SIG) fields for indicatingcertain PHY characteristics of the data unit. In one embodiment, the SIGfield includes information bits that specify the modulation type, codingrate, length, and other PHY characteristics of a data portion of thedata unit. Based on this information, a receiver can successfullydemodulate and/or decode the data portion of the data unit. In someembodiments, the SIG field additionally specifies one or more other PHYcharacteristics of the data unit, but without dedicating any additionalinformation bits for that purpose. For example, in an embodiment, one ormore OFDM symbols of the SIG field are modulated using a first symbolconstellation rotation (e.g., zero degrees) to indicate to a receiverthat the data unit is a single-user data unit, or with a second rotation(e.g., 90 degrees) to indicate to a receiver that the data unit is amulti-user data unit. In some embodiments, the SIG field specifies oneor more PHY characteristics of the SIG field itself without dedicatingany particular information bits to that purpose. For example, in anembodiment, one or more OFDM symbols of the SIG field are modulatedusing a first symbol constellation rotation or a second symbolconstellation rotation to indicate to a receiver that the SIG fieldutilizes a first number of information bits per OFDM symbol or a secondnumber of information bits per OFDM symbol (e.g., a lower or highernumber of bit repetitions, in an embodiment), respectively.

FIG. 1 is a block diagram of an example WLAN 10 including an AP 14,according to an embodiment. The AP 14 includes a host processor 15coupled to a network interface 16. The network interface 16 includes amedium access control (MAC) processing unit 18 and a physical layer(PHY) processing unit 20. The PHY processing unit 20 includes aplurality of transceivers 21, and the transceivers 21 are coupled to aplurality of antennas 24. Although three transceivers 21 and threeantennas 24 are illustrated in FIG. 1, the AP 14 can include differentnumbers (e.g., one, two, four, five, etc.) of transceivers 21 andantennas 24 in other embodiments.

The WLAN 10 further includes a plurality of client stations 25. Althoughfour client stations 25 are illustrated in FIG. 1, the WLAN 10 caninclude different numbers (e.g., one, two, three, five, six, etc.) ofclient stations 25 in various scenarios and embodiments. At least one ofthe client stations 25 (e.g., client station 25-1) is configured tooperate at least according to the long range communication protocol. Insome embodiments, at least one of the client stations 25 (e.g., clientstation 25-4) is a short range client station that is configured tooperate at least according to one or more of the short rangecommunication protocols.

The client station 25-1 includes a host processor 26 coupled to anetwork interface 27. The network interface 27 includes a MAC processingunit 28 and a PHY processing unit 29. The PHY processing unit 29includes a plurality of transceivers 30, and the transceivers 30 arecoupled to a plurality of antennas 34. Although three transceivers 30and three antennas 34 are illustrated in FIG. 1, the client station 25-1can include different numbers (e.g., one, two, four, five, etc.) oftransceivers 30 and antennas 34 in other embodiments.

In some embodiments, one, some, or all of the client stations 25-2,25-3, and 25-4 has/have a structure the same as or similar to the clientstation 25-1. In these embodiments, the client stations 25 structuredthe same as or similar to the client station 25-1 have the same or adifferent number of transceivers and antennas. For example, the clientstation 25-2 has only two transceivers and two antennas (not shown),according to an embodiment.

In various embodiments, the PHY processing unit 20 of the AP 14 isconfigured to generate data units conforming to the long rangecommunication protocol and having formats described below. Thetransceiver(s) 21 is/are configured to transmit the generated data unitsvia the antenna(s) 24. Similarly, the transceiver(s) 21 is/areconfigured to receive the data units via the antenna(s) 24. The PHYprocessing unit 20 of the AP 14 is also configured to process receiveddata units conforming to the long range communication protocol andhaving formats described below, according to various embodiments.

In various embodiments, the PHY processing unit 29 of the client device25-1 is configured to generate data units conforming to the long rangecommunication protocol and having formats described below. Thetransceiver(s) 30 is/are configured to transmit the generated data unitsvia the antenna(s) 34. Similarly, the transceiver(s) 30 is/areconfigured to receive data units via the antenna(s) 34. The PHYprocessing unit 29 of the client device 25-1 is also configured toprocess received data units conforming to the long range communicationprotocol and having formats described below, according to variousembodiments.

In some embodiments, the client station 25-1 can selectively operate ineither a normal mode (e.g., 2 MHz and wider bandwidths) or a lowbandwidth mode (e.g., 1 MHz bandwidth). In an embodiment, the same clockrate is used in either mode, with different IDFT sizes being utilized togenerate signals of different bandwidths (e.g., a 64-point or largerIDFT for the 2 MHz or wider bandwidths in normal mode, and a 32-pointIDFT for the 1 MHz bandwidth in low bandwidth mode). In some of theseembodiments, the low bandwidth mode is used as a control PHY. In otherof these embodiments, the low bandwidth mode is used to extend the rangeof the normal mode.

Low bandwidth mode communications are generally more robust than normalmode communications, having a sensitivity gain that supports extendedrange communications. For example, in an embodiment in which a normalmode utilizes a 64-point IDFT (e.g., for a 2 MHz bandwidth signal) togenerate normal mode data units, and in which a low bandwidth modeutilizes a 32-point IDFT (e.g., for a 1 MHz bandwidth signal) togenerate low bandwidth mode data units, the low bandwidth mode providesapproximately a 3 dB sensitivity gain. Moreover, in some embodiments,the low bandwidth mode introduces redundancy or repetition of bits intoat least some fields of the data unit to further reduce the data rateand further improve sensitivity gain. For example, in variousembodiments and/or scenarios, the low bandwidth mode introducesredundancy into the data portion and/or the SIG field of a low bandwidthmode data unit according to one or more repetition and modulation/codingschemes described below. In an embodiment where the low bandwidth modeincludes a 2× repetition of information bits, for example, a further 3dB sensitivity gain may be obtained. Still further, in some embodiments,the low bandwidth mode improves sensitivity gain by generating OFDMsymbols in accordance with the lowest data rate modulation and codingscheme (MCS) of the normal mode, or in accordance with an MCS lower thanthe lowest data rate MCS of the normal mode. As an example, in anembodiment, data units in normal mode are generated according to aparticular MCS selected from a set of MCSs, such as MCS0 (binary phaseshift keying (BPSK) modulation and 1/2 coding rate) to MCS9 (quadratureamplitude modulation (QAM) and 5/6 coding rate), with higher-order MCSscorresponding to higher data rates. In one such embodiment, for example,normal mode data units are generated using MCS0 or higher (MCS1, MCS2,etc.) without repetition of information bits, whereas low bandwidth modedata units are generated using MCS0 with 2× repetition of informationbits. Example embodiments of transmitters (e.g., within networkinterface 16 of AP 14, and/or network interface 27 of client station25-1) configured to generate normal and low rate/bandwidth mode dataunits are described in U.S. patent application Ser. No. 13/494,505, nowU.S. Pat. No. 8,826,106.

In the embodiments described below, low rate mode data units aretransmitted using a lower bandwidth than normal mode data units, and aretherefore referred to as “low bandwidth mode” data units. It isunderstood, however, that in other embodiments the data units of the lowrate mode are not transmitted over a lower bandwidth than normal modedata units. In some embodiments, for example, low rate mode data unitsare transmitted using the same bandwidth as the minimum bandwidth fornormal mode data units, and correspond to a lower data rate for otherreasons (e.g., MCS and/or repetition of information bits). Further,FIGS. 2, 3 and 5-7 show the normal mode data units as corresponding tobandwidths of 2 MHz or greater, and low bandwidth mode data units ascorresponding to a 1 MHz bandwidth. It is understood, however, thatnormal mode and low bandwidth mode data units of other embodiments maycorrespond to different bandwidths. It is also understood that thevarious fields shown, and/or the number of OFDM symbols per field shown,in FIGS. 2, 3 and 5-7 may differ in other embodiments.

FIG. 2 is a diagram of an example normal mode data unit 100 and lowbandwidth mode data unit 102, according to an embodiment. The data units100 and 102 are single-user data units. In one embodiment, and withreference to FIG. 1, the normal mode data unit 100 and low bandwidthmode data unit 102 are generated by network interface 16 of AP 14 andtransmitted to client station 25-1. The normal mode data unit 100includes an STF 110, a first LTF (LTF1) 112, a SIG field 114, anyadditional LTFs 116 (e.g., such that one LTF is included in data unit100 per spatial stream), and a data field 118. Similarly, the lowbandwidth mode data unit 102 includes an STF 120, a first LTF (LTF1)122, a SIG field 124, any additional LTFs 126, and a data field 128. Inan embodiment, the normal mode data unit 100 has the same format as anIEEE 802.11n data unit with a “Greenfield” preamble, and supports an MCSas low as MCS0. Moreover, in some of these embodiments, the lowbandwidth mode data unit 100 has a similar format, but utilizes 2×repetition of information bits in addition to MCS0. As a result of thelower data rate due to 2× repetition and a lower bandwidth (fewersubcarriers), the low bandwidth mode data unit 102 includes additionalOFDM symbols in some fields. For example, in the embodiment shown inFIG. 2, STF 110, LTF1 112 and SIG field 114 of normal mode data unit 100each include two OFDM symbols, whereas STF 120, LTF1 122 and SIG field124 of low bandwidth mode data unit 102 each include four or more OFDMsymbols. In the case of SIG field 124, for example, five or six OFDMsymbols may be needed in order to convey all of the necessary PHYinformation (e.g., MCS of data field 128, length of data field 128,etc.) to a receiver at the lower data rate. Various alternativeembodiments of the fields in the normal mode data unit 100 are describedin more detail in U.S. patent application Ser. No. 13/359,336, now U.S.Pat. No. 8,867,653, and various alternative embodiments of the fields inthe low bandwidth mode data unit 102 are described in more detail inU.S. patent application Ser. No. 13/366,064 (now U.S. Publication No.2012-0201316) and Ser. No. 13/494,505 (now U.S. Pat. No. 8,826,106).

In some embodiments, the normal bandwidth mode and the low bandwidthmode both support single-user and multi-user operation similar tosingle-user and multi-user operation as defined under IEEE 802.11ac.FIG. 3 is a diagram comparing an example single-user, normal mode dataunit 150 with an example multi-user, normal mode data unit 152,according to an embodiment. In one embodiment and scenario, and withreference again to FIG. 1, the single-user normal mode data unit 150 isgenerated by network interface 16 of AP 14 and transmitted to clientstation 25-1, and the multi-user, normal mode data unit 152 is generatedby network interface 16 of AP 14 and transmitted to two or more of theclient stations 25. The single-user, normal mode data unit 150 includesan STF 160, an LTF (LTF1) 162, two OFDM symbols (SIG1, SIG2) 164, 166 ofa SIG field, any additional LTFs 170, and a data field 172. In anembodiment, the single-user, normal mode data unit 150 is the same asthe normal mode data unit 100 of FIG. 2, with the two symbols of SIGfield OFDM symbols 164, 166 being shown separately rather than thesingle block 106 of FIG. 2.

The multi-user, normal mode data unit 152 includes an STF 180, a firstLTF (LTF1) 182, two OFDM symbols (SIGA1, SIGA2) 184, 186 of a first SIGfield, a multi-user STF (MUSTF) 190, multi-user LTFs (MULTFs) 192 forchannel estimation for all users, a second SIG field (SIGB) 194containing user-specific SIG field information, and a multi-user datafield 196 carrying data for all users. As seen in FIG. 3, the first andsecond OFDM symbols 164, 166 of the single-user, normal mode data unit150 are both modulated using quaternary binary phase shift key (QBPSK)modulation, whereas the multi-user, normal mode data unit 152 utilizesQBPSK modulation for the first SIGA field OFDM symbol 184, but binaryphase shift key (BPSK) modulation for the second SIGA field OFDM symbol186. FIG. 4 provides an illustration of a BPSK symbol constellation 200and a QBPSK symbol constellation 210. As seen in FIG. 4, QBPSKmodulation is identical to BPSK modulation, with the exception that thesymbol constellation 210 of QBPSK is rotated by 90 degrees relative tothe symbol constellation 200 of BPSK. Thus, in an embodiment, a receiverdetects whether a received data unit is single-user or multi-user (e.g.,has the format of data unit 150 or data unit 152) by detecting therotation of the symbol constellation in the second OFDM symbol of theSIG field. Various alternative embodiments of the fields in themulti-user, normal mode data unit 152 are described in more detail inU.S. patent application Ser. No. 13/494,505, now U.S. Pat. No.8,826,106, and also in U.S. patent application Ser. No. 13/464,467,“Preamble Designs for Sub-1 GHz Frequency Bands,” now U.S. PublicationNo. 2012-0294294, the disclosures of which are hereby incorporated byreference herein in their entireties.

FIG. 5 is a diagram comparing an example single-user, low bandwidth modedata unit 220 with an example multi-user, low bandwidth mode data unit222, according to an embodiment. In one embodiment and scenario, andwith reference again to FIG. 1, the single-user, low bandwidth mode dataunit 220 is generated by network interface 16 of AP 14 and transmittedto client station 25-1, and the multi-user, low bandwidth mode data unit222 is generated by network interface 16 of AP 14 and transmitted to twoor more of the client stations 25. The single-user, low bandwidth modedata unit 220 includes an STF 230, an LTF (LTF1) 232, a SIG field 234,any additional LTFs 236-1 through 236-N, and a data field 242. In anembodiment, the single-user, low bandwidth mode data unit 220 is thesame as the low bandwidth mode data unit 102 of FIG. 2.

The multi-user, low bandwidth mode data unit 222 includes an STF 250, afirst LTF (LTF1) 252, a first SIG field (SIGA) 254, a multi-user STF(MUSTF) 256, multi-user LTFs 260-1 through 260-N for channel estimationfor all users, a second SIG field (SIGB) 264 containing user-specificSIG field information, and a multi-user data field 266 carrying data forall users. In one embodiment, both the single-user and multi-user lowbandwidth mode data units 220, 222 are generated using a 32-point IDFT,with the SIG fields 234, 254 utilizing MCS0 with 2× bit repetition, andwith the SIGB field 264 of the multi-user data unit 222 utilizing MCS0without bit repetition. Moreover, in this embodiment, the MUSTF 256includes a single OFDM symbol having the same non-zero tones as the STF250 at the beginning of data unit 222, and the MULTFs 260-1 through260-N each include a single OFDM symbol such that all spatial streamsare trained for all users (e.g., using the same “P matrix” as defined inIEEE 802.11ac).

In some embodiments, low bandwidth mode data units utilize the same MCSand bit repetition (if any) regardless of whether the data units aresingle- or multi-user data units, but indicate to a receiver whether thedata unit is single-user or multi-user based on one or more symbolconstellation rotations within a SIG field (e.g., in a manner similar tothe single-user and multi-user normal mode data units 150, 152 of FIG.3, discussed above). For example, in one embodiment, one or moredesignated OFDM symbols within the SIG field (i.e., SIG field 234 ofsingle-user data unit 220 or SIGA field 254 of multi-user data unit 222)are modulated using either QBPSK or BPSK modulation to indicate to areceiver whether the data unit is single-user or multi-user,respectively (or vice versa).

FIGS. 6A and 6B illustrate two example embodiments in which the symbolconstellation rotation for one or more OFDM symbols in the SIG or SIGAfield is used to indicate whether a low bandwidth mode data unit issingle- or multi-user. Although FIGS. 6A and 6B show embodiments inwhich the SIG (or SIGA) field of the low bandwidth mode data unitincludes six OFDM symbols, other embodiments include more or fewer thansix OFDM symbols. In the embodiment of FIG. 6A, the SIG field 300corresponds to SIG field 234 of the single-user, low bandwidth mode dataunit 220 of FIG. 5, and the SIGA field 302 corresponds to SIGA field 234of the multi-user, low bandwidth mode data unit 222 of FIG. 5. In thesingle-user SIG field 300, all six OFDM symbols 310-1 through 310-6 useQBPSK subcarrier modulation. Conversely, in the multi-user SIGA field302, all six OFDM symbols 312-1 through 312-6 use BPSK subcarriermodulation. Thus, a receiver can detect whether a received low bandwidthmode data unit is single- or multi-user based on the detected symbolconstellation rotation of any or all of the OFDM symbols in the firstSIG field of the data unit.

In the embodiment of FIG. 6B, the SIG field 350 corresponds to SIG field234 of the single-user, low bandwidth mode data unit 220 of FIG. 5, andthe SIGA field 352 corresponds to SIGA field 234 of the multi-user, lowbandwidth mode data unit 222 of FIG. 5. In the single-user SIG field350, only the first OFDM symbol 360-1 uses QBPSK subcarrier modulation,while the remaining five OFDM symbols 360-2 through 360-6 use BPSKsubcarrier modulation. Conversely, in the multi-user SIGA field 352, allsix OFDM symbols 362-1 through 362-6 use BPSK subcarrier modulation.Thus, a receiver can detect whether a received low bandwidth mode dataunit is single- or multi-user based on the detected symbol constellationrotation of the first OFDM symbol in the first SIG field of the dataunit.

In alternative embodiments, any other combination of SIG field OFDMsymbols is selectively modulated using BPSK or QBPSK to indicate whetherthe low bandwidth mode data unit is single- or multi-user. Moreover, insome alternative embodiments, symbol constellation rotations are appliedto modulation types other than BPS K. For example, in some embodiments,higher-order modulation types are utilized for subcarriers of SIG fieldOFDM symbols (e.g., QPSK, 16-QAM, etc.), and are selectively rotated by90 degrees (or any other suitable amount) to indicate whether the lowbandwidth mode data unit is single- or multi-user.

While the modulation, coding, and/or bit repetition of a dataportion/field of a data unit is typically specified by PHY informationin the SIG field, a receiver generally must also know similar parametersfor the SIG field itself in order to demodulate and decode the PHYinformation contained in the SIG field. Thus, in some embodiments, theSIG field of single-user, low bandwidth mode data units use anMCS/repetition that is equal to the worst case MCS/repetition of thedata portion (e.g., MCS0, with 2× bit repetition), and therefore isknown a priori to the receiver, regardless of which MCS is utilized bythe data portion of the data unit. While this approach helps prevent thedemodulation and decoding of the SIG field from becoming a bottleneck tothe demodulation and decoding of the entire packet, such an approach canresult in an unnecessarily long SIG field and preamble in scenarioswhere a high quality communication channel exists. Accordingly, in analternative embodiment, the SIG field of a low bandwidth mode data unitcan selectively use one of at least two different MCSs (orMCS/repetition combinations). In one embodiment, for example, the SIGfield of a low bandwidth mode data unit uses MCS0 with 2× repetitionwhen the data portion of the low bandwidth mode data unit uses MCS0 with2× repetition, but uses MCS0 with no bit repetition when the dataportion uses any higher-order MCS or MCS/repetition combination (e.g.,MCS0 with no repetition, MCS1, MCS2, etc.).

In some embodiments, SIG fields using a lower-order MCS orMCS/repetition combination are longer (i.e., include more OFDM symbols)than SIG fields using a higher-order MCS or MCS/repetition combination,in order to accommodate all of the bits needed to specify the SIG fieldPHY information. For example, in one embodiment, SIG fields using MCS0without repetition are three OFDM symbols in length, while SIG fieldsusing MCS0 with 2× repetition are six OFDM symbols in length. Whenreferring herein to an embodiment in which the SIG field selectivelyuses one of two different MCSs or MCS/repetition combinations, the PHYmode corresponding to the higher-order MCS or MCS/repetition combinationis referred to as the “SIG-high” mode, and the PHY mode corresponding tothe lower-order MCS or MCS/repetition combination is referred to as the“SIG-low” mode.

While this approach allows for a shorter preamble (in some scenarios)while still preventing the demodulation/decoding of the SIG field frombecoming a bottleneck to the demodulation/decoding of the entire packet,it does generally require that a receiver be able to learn whether aparticular data unit corresponds to SIG-high or SIG-low mode. Thus, insome embodiments, the beginning one or more OFDM symbols of the SIGfield are modulated using a symbol constellation rotation that indicatesto a receiver whether the SIG field corresponds to SIG-high or SIG-lowmode. For example, in one embodiment, the beginning one or more OFDMsymbols of the SIG field are selectively modulated using either BPSK orQBPSK to differentiate between the two modes.

Further, in some embodiments, this approach is combined with theapproach described above (in connection with FIGS. 5 and 6) forindicating to a receiver whether a low bandwidth mode data unit is asingle- or multi-user data unit. In one embodiment, for example, thebeginning one or two OFDM symbols of the first SIG field (e.g., SIGfield 234 or SIGA field 254 in FIG. 5) selectively use BPSK or QBPSKmodulation to differentiate between SIG-high and SIG-low mode, and theremaining OFDM symbol(s) of the first SIG field selectively use BPSK orQBPSK modulation to differentiate between a single- and multi-user dataunit. In some embodiments, multi-user data units are not permitted ifthe data field uses the lowest MCS or MCS/repetition combination (e.g.,MCS0 with 2× repetition), and accordingly the first SIG field of amulti-user data unit (e.g., SIGA field 254 in FIG. 5) cannot utilize theSIG-low mode.

FIGS. 7A and 7B show two examples of this approach, for embodiments inwhich multi-user data units do not utilize SIG-low mode. Although FIGS.7A and 7B show an embodiment in which the SIG (or SIGA) field includesthree or six OFDM symbols, other embodiments include more or fewer OFDMsymbols. In the embodiment of FIG. 7A, the SIG field 400 and the SIGfield 402 correspond to SIG-low and SIG-high instances, respectively, ofSIG field 234 of the single-user, low bandwidth mode data unit 220 ofFIG. 5, and the SIGA field 404 corresponds to SIGA field 234 of themulti-user, low bandwidth mode data unit 222 of FIG. 5. The beginningtwo OFDM symbols 410-1, 410-2 of SIG field 400 use QBPSK modulation toindicate to a receiver that the SIG field 400 corresponds to SIG-lowmode (e.g., MCS0 with 2× repetition). In this embodiment, the SIG-lowmode rules out the possibility that SIG field 400 belongs to amulti-user data unit, and thus the remaining four OFDM symbols 410-3through 410-6 can be either BPSK modulated or QBPSK modulated. Forexample, in one embodiment, all six OFDM symbols 410-1 through 410-6 areQBPSK modulated.

The beginning two OFDM symbols 412-1, 412-2 of SIG field 402 instead useBPSK modulation to indicate to a receiver that the SIG field 402corresponds to SIG-high mode (e.g., MCS0 with no repetition). In thisembodiment, the receiver still needs to determine whether SIG field 402belongs to a single- or multi-user data unit. Thus, the third OFDMsymbol 412-3 is BPSK modulated to indicate to the receiver that the SIGfield 402 belongs to a single-user data unit.

The beginning two OFDM symbols 414-1, 414-2 of SIGA field 404 likewiseuse BPSK modulation to indicate to a receiver that the SIGA field 404corresponds to SIG-high mode (e.g., MCS0 with no repetition). Becausethe receiver also needs to determine whether SIGA field 404 belongs to asingle- or multi-user data unit, the third OFDM symbol 414-3 is QBPSKmodulated to indicate to the receiver that the SIGA field 404 belongs toa multi-user data unit.

In the alternative embodiment of FIG. 7B, the SIG field 450 and the SIGfield 452 correspond to SIG-low and SIG-high instances, respectively, ofSIG field 234 of the single-user, low bandwidth mode data unit 220 ofFIG. 5, and the SIGA field 454 corresponds to SIGA field 234 of themulti-user, low bandwidth mode data unit 222 of FIG. 5. The beginningOFDM symbol 460-1 of SIG field 450 uses QBPSK modulation to indicate toa receiver that the SIG field 450 corresponds to SIG-low mode (e.g.,MCS0 with 2× repetition). In this embodiment, the SIG-low mode rules outthe possibility that SIG field 450 belongs to a multi-user data unit,and thus the remaining five OFDM symbols 460-2 through 460-6 can beeither BPSK modulated or QBPSK modulated. For example, in oneembodiment, all six OFDM symbols 460-1 through 460-6 are QBPSKmodulated.

The beginning OFDM symbol 462-1 of SIG field 452 instead uses BPSKmodulation to indicate to a receiver that the SIG field 452 correspondsto SIG-high mode (e.g., MCS0 with no repetition). In this embodiment,the receiver still needs to determine whether SIG field 452 belongs to asingle- or multi-user data unit. Thus, the second OFDM symbol 462-2 isBPSK modulated to indicate to the receiver that the SIG field 452belongs to a single-user data unit. In this embodiment, the third OFDMsymbol 462-3 is not needed for determining whether the SIG field 452corresponds to SIG-high or SIG-low, or for determining whether the dataunit of SIG field 452 is single- or multi-user, and may be either BPSKor QBPSK modulated, according to different embodiments.

The beginning OFDM symbol 464-1 of SIGA field 454 likewise uses BPSKmodulation to indicate to a receiver that the SIGA field 454 correspondsto SIG-high mode (e.g., MCS0 with no repetition). Because the receiveralso needs to determine whether SIGA field 454 belongs to a single- ormulti-user data unit, the second OFDM symbol 464-2 is QBPSK modulated toindicate to the receiver that the SIGA field 454 belongs to a multi-userdata unit. In this embodiment, the third OFDM symbol 464-3 is not neededfor determining whether the SIGA field 454 corresponds to SIG-high orSIG-low, or for determining whether the data unit of SIGA field 454 issingle- or multi-user, and may be either BPSK or QBPSK modulated,according to different embodiments.

In alternative embodiments, any other combination of SIG field OFDMsymbols (preferably including the OFDM symbol at the beginning of theSIG field) is selectively modulated using BPSK or QBPSK to indicatewhether the SIG field corresponds to SIG-high or SIG-low mode, and anycombinations of one or more of the remaining OFDM symbols areselectively modulated using BPSK or QBPSK to indicate whether the lowbandwidth mode data unit is a single- or multi-user data unit. Moreover,in some alternative embodiments, symbol constellation rotations areapplied to modulation types other than BPSK. For example, in someembodiments, higher-order modulation types are utilized for subcarriersof SIG field OFDM symbols (e.g., QPSK, 16-QAM, etc.), and areselectively rotated by 90 degrees (or any suitable other amount) toindicate whether the SIG field corresponds to SIG-high or SIG-low mode,and/or the indicate whether the low bandwidth mode data unit is single-or multi-user.

FIG. 8 is a flow diagram of an example method 500 for generating a dataunit, according to an embodiment. The method 500 is implemented by thenetwork interface 16 of AP 14 or the network interface 27 of clientstation 25-1 of FIG. 1, in various embodiments and scenarios. In anembodiment, the method 500 is utilized by a network interface of adevice when the device is operating in a low bandwidth PHY mode.

At block 510, a first number of information bits per OFDM symbol isdetermined. The first number of information bits per OFDM symbolrepresents the number of information bits per OFDM symbol that will beused in generating a data field of a data unit. In various embodiments,a modulation type (BPSK, QPSK, 16-QAM, etc.), a coding rate (1/2, 3/4,5/6, etc.), a number of bit repetitions (no repetitions, 2×, 4×, etc.),and/or any other parameter that affects the number of information bitsper OFDM symbol, is/are determined for the data field at block 510. Inan embodiment, the first number of information bits per OFDM symbol isdetermined based on channel state information (e.g., signal-to-noiseratio, etc.), in order to ensure that the MCS and/or number ofrepetitions provide sufficiently robust performance in view of currentchannel conditions.

At block 520, a second number of information bits per OFDM symbol isdetermined based on the first number of information bits per OFDM symboldetermined at block 520. The second number of information bits per OFDMsymbol represents the number of information bits per OFDM symbol thatwill be used in generating a SIG field of the data unit. In variousembodiments, a modulation type (BPSK, QPSK, 16-QAM, etc.), a coding rate(1/2, 3/4, 5/6, etc.), a number of bit repetitions (no repetitions, 2×,4×, etc.), and/or any other parameter that affects the number ofinformation bits per OFDM symbol, is/are determined for the SIG field atblock 520. In an embodiment, the second number of information bits perOFDM symbol is set equal to the first number of information bits perOFDM symbol (e.g., the same MCS or MCS/repetitions) if the first numberof information bits per OFDM symbol (for the data field) corresponds toa lowest allowable MCS (or MCS/repetition combination), and is set to anumber corresponding to a second-lowest MCS (or MCS/repetitioncombination) otherwise.

At block 530, a symbol constellation rotation is determined based on thesecond number of information bits per OFDM symbol determined at block520. In one embodiment, for example, either BPSK or QBPSK is selected atblock 530 for at least a first OFDM symbol of the SIG field based on amodulation type, coding rate, and/or number of bit repetitions for theSIG field at block 520 (e.g., BPSK if a lower-order MCS orMCS/repetition combination will be used for the SIG field, or QBPSK if ahigher-order MCS or MCS/repetition combination will be used for the SIGfield).

At block 540, a SIG field of the data unit is generated according to thesecond number of information bits per OFDM symbol (determined at block520), with at least one OFDM symbol of the SIG field being generatingaccording to the symbol constellation rotation determined at block 530.For example, in one embodiment and scenario where BPSK modulation, 1/2rate coding, and 2× repetition is determined for the SIG field at block520, that MCS and bit repetition is used to generate each OFDM symbol ofthe SIG field at block 540, with at least one OFDM symbol (e.g., thebeginning one or more OFDM symbols of the SIG field) being generatedusing either BPSK or QBPSK modulation depending on whether a zero degreeconstellation rotation or a 90 degree constellation rotation,respectively, was determined at block 530. The SIG field provides areceiver with PHY information for interpreting at least the data fieldof the data unit (e.g., MCS of the data field, length of the data field,etc.).

At block 550, a data field of the data unit is generated according tothe first number of information bits per OFDM symbol (determined atblock 510). For example, in one embodiment and scenario where QPSKmodulation and rate 3/4 coding is determined for the data field field atblock 510, the OFDM symbols of the data field are generated using QPSKmodulation and rate 3/4 coding.

In some embodiments, the method 500 includes additional blocks not shownin FIG. 8. In one embodiment, for example, the method 500 includes ablock in which an additional symbol constellation rotation is determinedbased on whether the data unit being generated is a single- ormulti-user data unit. In this embodiment, generating the SIG field atblock 540 further includes the generation of at least one other OFDMsymbol according to the additional determined rotation.

FIG. 9 is a flow diagram of an example method 600 for receiving andprocessing a data unit, such as a data unit generated according tomethod 500 of FIG. 8, according to an embodiment. The method 600 isimplemented by the network interface 27 of client station 25-1 or thenetwork interface 16 of AP 14 of FIG. 1, in various embodiments andscenarios.

At block 610, a data unit that includes a SIG field and a data field isreceived via a communication channel and one or more antennas. In anembodiment, the data unit is a low bandwidth mode data unit such as thesingle-user data unit 220 or multi-user data unit 222 of FIG. 5, forexample. Moreover, in an embodiment, the data unit includes a SIG fieldsimilar to one of the SIG (or SIGA) fields of FIG. 7A or 7B. In someembodiments, however, the SIG field does not indicate whether the dataunit is single- or multi-user. The SIG field provides PHY informationthat allows a receiver to interpret at least the data field of the dataunit. In some embodiments, the data unit includes other fields as well,such as an STF and LTF preceding the SIG field.

At block 620, a symbol constellation rotation of at least a first OFDMsymbol in the SIG field of the data unit received at block 610 isdetected. In one embodiment, the first OFDM symbol is the beginning OFDMsymbol of the SIG field (i.e., the first sequentially within the SIGfield). In other embodiments, the first OFDM symbol is later in thesequence of SIG field OFDM symbols. In some embodiments, the symbolconstellation rotation is detected over a span of two or more OFDMsymbols of the SIG field (e.g., the beginning two OFDM symbols of theSIG field, in an embodiment). In some embodiments, a receiver detectswhether a modulation type has a zero degree or 90 degree constellationrotation. For example, in one embodiment, whether the first OFDM symbol,or multiple OFDM symbols, of the SIG field is/are BPSK or QBPSKmodulated is detected at block 620.

At block 630, a number of information bits per OFDM symbol in the SIGfield is determined, based at least in part on the symbol constellationrotation detected at block 620. For example, in various embodiments, amodulation type, coding rate, and/or number of bit repetitions isdetermined at block 630. As one more specific example, it is determinedat block 630 whether the SIG field utilizes MCS0 with 2× repetition, orMCS0 without bit repetition, based on whether BPSK or QBPSK modulationis detected (at block 620) in one or more OFDM symbols.

At block 640, the SIG field of the data unit received at block 610 isprocessed (e.g., demodulated and decoded) according to the number ofinformation bits per OFDM symbol in the SIG field (e.g., the determinedmodulation type, coding rate, and/or bit repetition, in variousembodiments), as determined at block 630.

At block 650, the data field of the data unit received at block 610 isprocessed (e.g., demodulated and decoded) according to the PHYinformation, included in the SIG field processed at block 640, forinterpreting the data unit.

In some embodiments, the method 600 includes additional blocks not shownin FIG. 9. In one embodiment, for example, the method 600 includes afirst additional block in which another symbol constellation rotation isdetected in at least a second OFDM symbol of the SIG field, and a secondadditional block in which it is determined, based on that detectedrotation, whether the data unit received at block 610 is a single- ormulti-user data unit.

At least some of the various blocks, operations, and techniquesdescribed above may be implemented utilizing hardware, a processorexecuting firmware instructions, a processor executing softwareinstructions, or any combination thereof. When implemented utilizing aprocessor executing software or firmware instructions, the software orfirmware instructions may be stored in any computer readable memory suchas on a magnetic disk, an optical disk, or other storage medium, in aRAM or ROM or flash memory, processor, hard disk drive, optical diskdrive, tape drive, etc. Likewise, the software or firmware instructionsmay be delivered to a user or a system via any known or desired deliverymethod including, for example, on a computer readable disk or othertransportable computer storage mechanism or via communication media.Communication media typically embodies computer readable instructions,data structures, program modules or other data in a modulated datasignal such as a carrier wave or other transport mechanism. The term“modulated data signal” means a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia includes wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, radio frequency,infrared and other wireless media. Thus, the software or firmwareinstructions may be delivered to a user or a system via a communicationchannel such as a telephone line, a DSL line, a cable television line, afiber optics line, a wireless communication channel, the Internet, etc.(which are viewed as being the same as or interchangeable with providingsuch software via a transportable storage medium). The software orfirmware instructions may include machine readable instructions that,when executed by the processor, cause the processor to perform variousacts.

When implemented in hardware, the hardware may comprise one or more ofdiscrete components, an integrated circuit, an application-specificintegrated circuit (ASIC), etc.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, changes, additions and/or deletions may bemade to the disclosed embodiments without departing from the scope ofthe claims.

What is claimed is:
 1. A method comprising: receiving a data unit thatincludes (i) a data field and (ii) a preamble that includes a firstsignal (SIG) field, wherein the first SIG field provides information forinterpreting the data field; detecting a symbol constellation rotationof an orthogonal frequency division multiplexing (OFDM) symbol of thefirst SIG field; determining, based at least in part on the detectedsymbol constellation rotation, whether the preamble is arrangedaccording to a short format or a long format, wherein when the preambleis arranged according to the long format, the data unit includes asecond signal field that is omitted from the data unit when the preambleis arranged according to the short format; processing at least the firstSIG field of the preamble; and processing the data field of the dataunit according to the information for interpreting the data field asprovided in at least the first SIG field of the preamble.
 2. A methodaccording to claim 1, wherein detecting the symbol constellationrotation includes detecting whether the OFDM symbol is binary phaseshift key (BPSK) modulated or quaternary BPSK (QBPSK) modulated.
 3. Amethod according to claim 2, wherein determining, based at least in parton the detected symbol constellation rotation, whether the preamble isarranged according to the short format or the long format comprises:when it is detected that the OFDM symbol in the first SIG field is BPSKmodulated, determining that the preamble is arranged according to thelong format, and when it is detected that the OFDM symbol in the firstSIG field is QBPSK modulated, determining that the preamble is arrangedaccording to the short format.
 4. A method according to claim 1, whereinwhen the preamble is arranged according to the long format, the dataunit further includes at least one short training field not included inthe data unit when the preamble is arranged according to the shortformat.
 5. An apparatus comprising: a network interface configured toreceive a data unit that includes (i) a data field and (ii) a preamblethat includes a first signal (SIG) field, wherein the first SIG fieldprovides information for interpreting the data field; detect a symbolconstellation rotation of an orthogonal frequency division multiplexing(OFDM) symbol of the first SIG field; determine, based at least in parton the detected symbol constellation rotation, whether the preamble isarranged according to a short format or a long format, wherein when thepreamble is arranged according to the long format, the data unitincludes a second signal field that is omitted from the data unit whenthe preamble is arranged according to the short format; process at leastthe first SIG field of the preamble; and process the data field of thedata unit according to the information for interpreting the data fieldas provided in at least the first SIG field of the preamble.
 6. Anapparatus according to claim 5, wherein the network interface isconfigured to detect the symbol constellation rotation at least bydetecting whether the OFDM symbol is binary phase shift key (BPSK)modulated or quaternary BPSK (QBPSK) modulated.
 7. An apparatusaccording to claim 6, wherein the network interface is configured todetermine based at least in part on the detected symbol constellationrotation, whether the preamble is arranged according to the short formator the long format at least by: when it is detected that the OFDM symbolin the first SIG field is BPSK modulated, determining that the preambleis arranged according to the long format, and when it is detected thatthe OFDM symbol in the first SIG field is QBPSK modulated, determiningthat the preamble is arranged according to the short format.
 8. Anapparatus according to claim 5, wherein when the preamble is arrangedaccording to the long format, the data unit further includes at leastone short training field not included in the data unit when the preambleis arranged according to the short format.
 9. A method for generating adata unit, the method comprising: determining whether a preamble of thedata unit is to be arranged according to a short format or a longformat; determining, based on whether the preamble is to be arrangedaccording to the short format or the long format, a symbol constellationrotation for an orthogonal frequency division multiplexing (OFDM) symbolof a first signal (SIG) field of the preamble of the data unit;generating the first SIG field of the preamble of the data unit, whereinthe first SIG field provides information for interpreting a data fieldof the data unit, and generating the first SIG field includes generatingthe OFDM symbol of the first SIG field according to the first symbolconstellation rotation; generating the preamble of the data unitaccording to the short format or the long format, wherein the preambleis generated to include at least the first SIG field; and generating thedata unit to include at least the preamble and the data field, whereinwhen the preamble is generated according to the long format, generatingthe data unit to further include a second signal field, and when thepreamble is generated according to the short format, generating the dataunit to omit the second signal field.
 10. A method according to claim 9,wherein determining the symbol constellation rotation includes selectingbinary phase shift key (BPSK) modulation or quaternary BPSK (QBPSK)modulation for the OFDM symbol of the first SIG field.
 11. A methodaccording to claim 10, wherein determining the symbol constellationrotation includes: when it is determined that the preamble is to bearranged according to the long format, selecting BPSK modulation for theOFDM symbol of the first SIG field, and when it is determined that thepreamble is to be arranged according to the short format, selectingQBPSK modulation for the OFDM symbol of the first SIG field.
 12. Amethod according to claim 9, wherein when the preamble is generatedaccording to the long format, generating the data unit to furtherinclude a short training field not included in the data unit when thepreamble is generated according to the short format.
 13. An apparatuscomprising: a network interface configured to determine whether apreamble of a data unit is to be arranged according to a short format ora long format; determine, based on whether the preamble is to bearranged according to the short format or the long format, a symbolconstellation rotation for an orthogonal frequency division multiplexing(OFDM) symbol of a first signal (SIG) field of the preamble of the dataunit; generate the first SIG field of the preamble of the data unit,wherein the first SIG field provides information for interpreting a datafield of the data unit, and generating the first SIG field includesgenerating the OFDM symbol of the first SIG field according to the firstsymbol constellation rotation; generate the preamble of the data unitaccording to the short format or the long format, wherein the preambleis generated to include at least the first SIG field; and generate thedata unit to include at least the preamble and the data field, whereinwhen the preamble is generated according to the long format, the dataunit is generated to further include a second signal field, and when thepreamble is generated according to the short format, the data unit isgenerated to omit the second signal field.
 14. An apparatus according toclaim 13, wherein the network interface is configured to determine thesymbol constellation rotation at least by selecting binary phase shiftkey (BPSK) modulation or quaternary BPSK (QBPSK) modulation for the OFDMsymbol of the SIG field.
 15. An apparatus according to claim 14, whereinthe network interface is configured to determining the symbolconstellation rotation at least by: when it is determined that thepreamble is to be arranged according to the long format, selecting BPSKmodulation for the OFDM symbol of the first SIG field, and when it isdetermined that the preamble is to be arranged according to the shortformat, selecting QBPSK modulation for the OFDM symbol of the first SIGfield.
 16. An apparatus according to claim 15, wherein the networkinterface is configured to when the preamble is generated according tothe long format, generate the data unit to further include a shorttraining field not included in the data unit when the preamble isgenerated according to the short format.